The present invention relates to a method and apparatus for delaying the separation of a boundary layer flowing over a solid surface to higher adverse pressure gradients associated with larger divergence of the flow. The invention is especially applicable to wings, for increasing the maximum angle of attack and thereby the lift of the wing, and is therefore described below particularly with respect to this application. The invention, however, may be advantageously used in other applications, for example in diffusers, in order to increase the permissible angle of divergence of the diffuser for the purpose of slowing the fluid flow and recovering pressure.
The loss of lift, commonly referred to in the literature as "stall", results from the separation of the boundary layer in the airstream flowing over the wing as a result of an adverse-pressure gradient which cannot be negotiated by the boundary layer. Such a separation is induced by an increase in the angle of attack of the wing above a predetermined maximum, and limits the maximum lift of a wing of given dimensions.
A similar separation of the boundary layer occurs in diffusers having an enlargement for slowing the fluid flow and recovering pressure. Thus, the angle of the diffuser surface at the enlargement is limited, typically to a 60 equivalent conical angle, in order to prevent separation of the boundary layer.